Conventional devices perform a method for selective catalytic reduction (SCR) has proven successful for decreasing the nitrogen oxides contained in the exhaust gas of an internal combustion engine. In this method nitrogen oxides are reacted together with ammonia in a selective catalytic converter, yielding nitrogen and water. The reducing agent required for catalytic conversion of the nitrogen oxides is carried in the vehicle in a reservoir tank in the form of an aqueous urea solution rather than ammonia, and is injected by way of an injection device into the exhaust gas of the internal combustion engine in order to carry out the desired reaction with the exhaust gas downstream in a catalytic converter. For better mixing of the exhaust gas with the exhaust gas post-treatment agent it is also known to provide, downstream from the metering device, a mixing apparatus having multiple air guidance elements; this apparatus, constituting a static mixer, promotes mixing of the exhaust gas post-treatment agent with the exhaust gas. A corresponding apparatus is described, for example, in German Patent Application No. DE 10 2006 011 890 A1. German Patent Application No. DE 100 60 808 A1 also describes a corresponding mixing apparatus, although it is disposed upstream from the metering apparatus.